An ab initio calculation of electronic excitation energies based on the GW approximation is performed by using the all-electron mixed-basis approach. The generalized plasmon-pole (GPP) model is used to determine the electron self-energyand the results are compared with our previous results using the full frequency integration (S. Ishii, et al., Phys. Rev. B 63 155104 (2001)). It is found that the two methods with and without the GPP model agrees well in the case of sodium clusters.